Limitation of carbon in single crystal or columnar-grained nickel base superalloys

ABSTRACT

THE NICKEL-BASE SUPERALLOYS WHICH ARE FABRICATED BY CONTROLLED UNIDIRECTIONAL SOLIDIFICATION INTO ANISOTROPIC ARTICLES, PARTICULARLY IN SINGLE CRYSTAL OR COLUMNAR-GRAINED FORM, ARE LIMITED TO A MAXIMUM CARBON CONTENT OF ABOUT 100 PARTS PER MILLION TO PROVIDE SUPERIOR FATIGUE RESISTANCE.

United States Patent 3,567,526 LIMITATION OF CARBON IN SINGLE CRYSTAL ORCOLUMNAR GRAINED NICKEL BASE SUPERALLOYS Maurice L. Gell, Rocky Hill,and Gerald R. Leverant,

Wapping, Conn., assignors to United Aircraft Corporation, East Hartford,Conn. No Drawing. Filed May 1, 1968, Ser. No. 725,889 Int. Cl. C22c19/00 US. Cl. 148-32.5 3 Claims ABSTRACT OF THE DISCLOSURE Thenickel-base superalloys which are fabricated by controlledunidirectional solidification into anisotropic articles, particularly insingle crystal or columnar-grained form, are limited to a maximum carboncontent of about 100 parts per million to provide superior fatigueresistance.

BACKGROUND OF THE INVENTION This invention relates primarily to thestrong, hightemperature alloys, particularly the nickel-basesuperalloys, and to casting compositions for use in the controlledsolidification of these superalloys.

A nickel-base superalloy is typically a nickel/ chromium solid solutionhardened by the additions of aluminum, titanium and/ or niobium toprecipitate the intermetallic compound Ni (M) where M is aluminum,titanium, niobium or a combination thereof. Commercial superalloys, inaddition, usually contain cobalt to raise the solvus temperature of theprecipitate, refractory metal additions for solution strengthening, andcarbon, boron and zirconium to promote creep-rupture ductility.

Carbon has traditionally been employed as a necessary alloying additionin the polycrystalline nickel-base superalloys. Most of the carboncombines with other alloying ingredients such as titanium, chromium,zirconium, niobium and tungsten to form metal carbides in a variety ofphases (MC, M C M C and M C the particular phase or phases presentdepending on the composition and heat treatment. Since the hightemperature stress-rupture life and ductility of the superalloyscontaining carbides are superior to those without the carbides, thecarbon alloying additions have, in the past, generally been consideredessential in the polycrystalline superalloys. This is confirmed by arecent study by J. P. Stroup and L. A. Pugliese in Metal Progress, vol.93, pages 96-100 (February 1968). And the carbon requirement remains afact despite certain studies which have indicated that a limited seriesof nickel-base alloys have been found to have improved impact resistanceat low carbon levels. See, for example, British Pat. No. 1,033,715.

Recently there have been developed certain casting techniques concernedwith the controlled or unidirectional solidification of the superalloysinto articles of columnargrained or single crystal form. The particularpreferred directional solidification techniques are discussed in thepatent to VerSnyder 3,260,505, and in the copending application ofPiearcey, Ser, No. 540,114, filed Feb. 17, 1966, now Pat. No. 3,494,709entitled, Cast Metal Part and Process and Apparatus Therefor. Both thepatent and the copending application share a common assignee with thepresent invention.

Generally speaking, the alloys heretofore utilized in the variousdirectional solidification processes have been of the same compositionas those alloys utilized in the conventional casting techniques and,accordingly, have included the usual carbon alloying additions.Representative nickel-base superalloys and their conventionalcompositions are set forth below.

ice

Alloy designation Composition (percent by weight) MAR-M20O 9 Cr, 10 O0,2 Ti, 5 Al, 12.5 W, 1 Nb, .015 B, .05 Zr,

.15 0, balance Ni.

Udimet 700 14.6 Cr, 15.3 Co, 3.4 Ti, 4.3 Al, 4. 1 M0, .016 B, .07 0

balance Ni.

IN 10 Cr, 15 Co, 4.5 Ti, 5.5 A1, 3 Mo, .75 V, .015 B, .075 Zr, .17 0,balance Ni.

13-1900 8 Cr, 10 Co, 1 Ti, 6 A1, 6 M0, 4.3 Ta, .15 B, .07 Zr,

.11 0, balance Ni.

1 Sigma tree.

SUMMARY OF THE INVENTION Briefly stated, the present invention relatesto the discovery that the carbon content of the columnar-grained andsingle crystal castings of the nackel-base superalloys should be limitedto a maximum of about 100 parts per million (.01 percent by weight).

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has recently been found thatthe physical properties of the directionally-solidified, nickel-basesuperalloys, particularly the fatigue resistance thereof, are adverselyaffected to a significant extent by the presence in these alloys of alarge MC-type carbide phase. An examination of the microstructure of thenickel-base superalloys of conventional composition, in bothcolumnar-grained and single crystal form, has revealed that the MC-typecarbides are much larger in these forms than in the conventionalcastings. Further, these carbides can exist as interconnected networks.These large MC-type carbides result from the slower solidification rateand shallow thermal gradient at the solidification front used in thecasting of the anisotropic structures. It was also discovered that manyof these carbides were precracked parallel to their longest dimension.Since the cracks were present in the materials immediately aftercasting, it is evident that they were formed during solidification ofthe alloys. The precracking of the carbides had not been observedpreviously, and an extremely careful mechanical polish or electropolishwas required to reveal them.

.The plastic deformation and matrix cracking of the as-cast orheat-treated structures, in tensile and fatigue tests, were found toinitiate at the tips of the cracks already present in the MC-typecarbides, and it was shown that the fatigue life of these as-cast alloysis a very sensitive function of carbide size. It was demonstrated, forexample, that the fatigue lives of specimens taken from the tops of castbars were reduced by a factor of more than three hundred when comparedto samples taken from the bottom or middle of the same bars where thecarbides were smaller because of a faster solidification rate and asteeper thermal gradient at the solidification front. This is clearlydemonstrated by the data of Table I.

It has also been found that the creep-rupture lives of specimens takenfrom the tops of cast bars is approximately one-half that of specimenstaken from the lower portion of the same bars.

The fatigue properties of the nickel-base superalloys incolumnar-grained and single crystal forms can be substantially improvedby eliminating the MC-type carbides from the alloy microstructure. Thisis accomplished by limiting the carbon content of the alloy to a maximumof about 100 parts per million (0.0-1 percent by weight). Such carbonlevels may be achieved by melting and casting the superalloys undervacuum without intentionally adding carbon to the alloy. Any carbon thatis present is accordingly the result of carbon impurities present in thevarious elements used in the alloying process. With the carbides nolonger required or desired in the alloy, small adjustments may be madeto the nominal composition of the various alloys to take into accountthe fact that certain amounts of the elements are no longer utilized information of the carbides. Typical low carbon MAR- M200 and UDIMET 700compositions are compared in Table -H with the corresponding alloys attheir normal chemistry.

TABLE II nickel-base superalloys in columnar-grained and single crystalform by maintaining the carbon content below' 100 parts per million tothereby eliminate the precracked MC- type carbides which act as thesites for slip and crack initiation in fatigue. While the inventiorr hasbeen described in connection with certain examples for the purposes erillustration, no limitation is intended thereby. Numerous modificationsand additions will be evident to those skilled in the art from theteachings herein In accordance with the true spirit of the invention,the scope thereof will be measured not by the illustrative material butby the appended claims.

What is claimed is; 7

1. In those processes wherein a nickel-base superalloy comprising afirst phase consisting principally of a nickel/ 1 OWTiNbZrCoCrAlBMo NiMAR-M200:

Normal 15 12. 5 2. 1. 0 I 10. 0 9. O0 5. 00 015 Bal. Low carbon 01 12.l 1. 7 6 04 10. 0 9. 00 5. 00 015 Bal. UDIMET 700 (sigma free) a Normal07 3 4 15. 3 14. 6 4-. 3 016 4. 4 Bal.

Low carbon .s 01 3 2 15. 3 14. l 4. 3 4. 2 Bal.

I Single crystal articles prepared from the MAR-M200 alloy of the lowcarbon formulation have displayed superior fatigue life as compared tothe corresponding articles containing the MC-type carbides. Examinationhas shown no MC-type carbide and, hence, no carbide cracking in the lowcarbon alloy. This is supported by the comparative data of Table III. t

e TABLE 11f Low cycle fatigue of nickel-base superalloys at 1,40 F. attotal strain range of 1.6 percent] It is evident that the severity ofthe problem with precracked carbides in controlled solidificationcasting is a function of both the carbon content of the alloy and thesolidification parameters in the casting process, insofar as they affectcarbide size, type and distribution in the cast article. Below about 600parts per million at the normal solidification rates, an improvement inthe properties of theldirectionally solidified article will be found asthe carbon content of the alloy is reduced and the carbide size islessened whereby the 'precraclc ng becomes less frequent and morescattered. Below 100 parts per millioncarbon, no MC-type carbides areformed.

By the present invention there has been provided means for substantiallyimproving the fatigue'properties of the chromium solid solution and asecond phase consisting of an intermetallic compound of the compositionNi (M), where M is aluminum, titanium, niobium or a combination thereofis cast into a columnar-grained or single'crystal article bycontrolledunidirectional solidification, the improvement which compriseslimiting the carbon content of the superalloy to a maximum, by weight,of. about 100 parts per million. 7

2. A unidirectionally-solidified article which consists of a first phaseconsisting principally of a nickel/ chromium solid solution and a secondphase consisting of an intermetallic compound of the composition Ni (M),where M is aluminum, titanium, niobium or a combination there: of and amaximum carbon content of about 0.01 percent, the article being furthercharacterized by the substantial absence of large carbides in themicrostructure.

3. A unidirectional cast article of pronounced anistropy which consistsof afirst phase principally consisting ola nickel/chromium solidsolution and a second phase consisting of an intermetallic compound ofnickel with aluminum arid titanium and a maximum carbon content of about0.01 percent, the article being characterized by the substantial absenceof carbides in the microstructure References Cited UNITED STATES PATENTS2,766,156 10/1956- Hetteridge et al. 148-162 3,260,505 7/1966 VerSnyder-l71 3,322,534 5/1967 Shaw et al. 75 171 3,376,132 4/1968 Shaw et 6175-171 3,457,066 7/1969 .Pohlman; et al. 75-171 3,494,709 2/1970Piearcey 75 171 RICHARD o. DEAN, Primary Examiner US. 01. X.R.

